You were once the size of a strawberry — wide-nosed, wide-eyed, large-headed, with a bowed neck that made you look, quite accurately, like a small levitating alien. Over the following weeks, your chin would grow, your neck would emerge, your eyelids would close over dark eyes. You would begin, gradually, to look human.
But beneath that visible transformation, something far more intricate was already underway. Your body was making some of its most consequential decisions — about sex, about plumbing, about the architecture that would sustain you for the rest of your life. And it was doing so in ways that are strange, improvised, and quietly remarkable.
The Weeks Before You Had a Sex
For the first several weeks of development, there are no differences between male and female embryos. None at all. The internal architecture is identical — two sacs, each connected to two small ducts, waiting to be assigned a purpose.
This is, incidentally, why men have nipples. Not because they serve any function, but because they are already in place before sex differentiation begins. The body builds first and decides later.
The decision, when it comes, is driven by a single gene on the Y chromosome — a gene called SRY.
The Y chromosome is, by any measure, a diminished thing. The X chromosome, carried by both men and women, contains somewhere between 800 and 900 genes. The Y probably carries as few as fifty or sixty. It looks, in comparison, almost forlorn. But its decisive contribution is that one gene: SRY.
The protein produced by SRY does not do very much on its own. What it does is act as a switch — activating a cascade of other genes scattered across different chromosomes. Together, these genes build the testicles. The testicles then begin sending hormonal instructions outward into the developing body. The first hormone causes one of the two ducts to be reconstructed — in women, this duct stays unchanged and eventually becomes the ovary and uterus. A second hormone redirects the remaining duct to become part of the sperm delivery system. Then, a little later, the testicles begin producing testosterone in large quantities — a signal that travels through the entire body, effectively announcing: become male.
Without SRY, none of this happens. The default path, in its absence, is female. Ovaries form automatically. The ducts become ovaries and uterus. The body, absent instruction, chooses a direction.
When the Message Cannot Be Heard
The testosterone signal only works if the cells can receive it. Each cell carries receptors on its surface designed to catch these hormonal messages. If those receptors are faulty, the testicles can produce testosterone continuously and the body will not respond. It simply cannot hear the instruction.
In this case — despite the presence of a Y chromosome and functioning testicles — the external body develops female characteristics. The genitals form as female, because the fate of external sex organs is determined by the testosterone signal. Internally, however, the ovaries and uterus are absent, because the testicle already sent the hormonal instruction to destroy the duct that would have formed them — and that instruction, unlike the testosterone, was received.
The body, in other words, exists in a kind of biological contradiction: external female features, internal gland functioning as a testicle, no uterus.
All Females Are a Genetic Patchwork
For embryos destined to be female, a different kind of decision must be made. Every female cell carries two X chromosomes — one inherited from the mother, one from the father. Having two copies creates a problem: the cell would produce a double dose of everything encoded in the X chromosome. Too many cooks, too much output.
The solution is elegant and permanent: one of the two X chromosomes is switched off in every cell. Which one gets switched off is entirely random. Some cells silence the maternal X; others silence the paternal X. The decision, once made in each cell, is fixed for all that cell’s descendants.
The result is that every female body is a mosaic — a patchwork of cells running on slightly different genetic instructions. Some patches express the maternal X; others express the paternal. This genetic variation, distributed across the body, is why all females are, at a cellular level, unique composites rather than uniform expressions of a single genome.
How Temperature Decides Sex in Alligators
Sex determination by chromosome is not a universal rule. Alligators do not use it at all.
For alligators, everything is decided by temperature during the first three weeks of incubation. Keep the egg below thirty degrees Celsius, and a female will develop. Raise it above thirty-four degrees, and a male will hatch. Land the temperature somewhere in the middle, and you will get a mixed result, with females predominating.
There is something almost philosophical about this. The most fundamental biological characteristic — sex — is determined, in one of Earth’s oldest animal lineages, not by genetics but by the ambient warmth of a nest.
Why Your Testicles Had to Travel
If a Y chromosome is present and the testosterone signal reaches where it needs to go, a penis develops — growing from a small bud that in female embryos becomes a clitoris. By roughly three months after conception, this bud has grown large enough for the sex of the foetus to be visible externally.
But the testicles, at this point, are still inside the body. They remain there until the seventh month, when they begin their gradual journey — first downward toward the abdomen, then into the scrotum.
Fish, by contrast, keep their testicles beside their hearts for their entire lives. This works for fish because they are cold-blooded, their internal temperature matching their surroundings. Sperm cells, however, do not survive well in heat. Human internal organs run warm. So the testicles live outside the body, in a small pouch capable of contracting or expanding in response to external temperature — continuously adjusting to keep the sperm at their optimum environment.
It is one of biology’s more unglamorous solutions to a genuine engineering problem.
Three Attempts at a Kidney
The kidneys are built at the same time as the genitals and originate from the same cell structures. Their development is, by any standard, extraordinarily convoluted — a record of the body trying something, abandoning it, trying again, and finally succeeding on the third attempt.
The first kidneys appear in the third week of development, right beside the neck. They are a small cluster of tubes, functionally useless. They disappear quickly.
A second pair forms further down the back shortly afterward. These are more recognizable — similar in structure to the kidneys found in fish and amphibians. They actually function briefly inside the womb. In female embryos, they eventually disappear entirely. In males, some of their cells remain behind and become part of the genitals.
In the fifth week, construction of the final kidneys begins. But they form in the wrong location, and must migrate — first downward toward the pelvis to connect to the bladder, then upward to their permanent position on either side of the spinal column, roughly level with the lowest ribs.
The kidneys you use every day are the result of a process that involved building them twice wrong before building them right, in the wrong place, and then moving them to where they needed to be.
The Salt That Keeps You Alive
Once working, the kidneys perform one of the body’s most essential and least acknowledged tasks: maintaining the precise balance of salt and water in every cell.
Your heart cannot beat without salt. Your nerve cells cannot generate electrical signals without it. Without salt, you cannot think, feel, or move. You would be, as the biology puts it plainly, stone dead.
The importance of salt balance becomes clear when you consider what happens when it goes wrong. A cell placed in water saltier than itself has no choice but to donate its own water to the surrounding solution. It shrinks, shrivels, and collapses — resembling, eventually, a raisin. Bacterial cells are equally vulnerable, which is why salt has been used to preserve food across human history. Salt pulls the water out of bacteria just as it pulls it from any other cell.
Your kidneys prevent you from becoming a collection of several trillion floppy raisin-cells. They monitor salt and water concentrations continuously, adjust what is excreted, and maintain the precise internal environment that every cell depends on.
You Were Drinking Your Own Urine Before You Were Born
In the womb, the kidneys do not yet need to manage waste on their own. All of that is handled by the mother’s blood and her kidneys, which filter the foetus’s waste on its behalf.
But the foetal kidneys do not sit idle. By the ninth week, they have already begun producing urine. By the tenth, the foetus is drinking small amounts of amniotic fluid and urinating it back out — rehearsing, in the quiet of the womb, the functions they will need to perform independently from the moment of birth.
It is, on reflection, a reasonable arrangement. You practice the job while someone else does the real work. Then, when the time comes, you take over.
Your body has been preparing for independence since before it could be told apart from any other human being. It built itself three times, made its most fundamental decisions randomly and irreversibly, and arrived — somehow — exactly where it needed to be.
Source : The Making of You: A Journey from Cell to Human by Katharina Vestre
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